This invention relates to optical amplifiers employing optically pumped optically amplifying waveguide, typically optically amplifying optical fibre. One of the known types of optical amplifier fibre is erbium doped optical fibre, and it is known that this can be pumped at 1480 nm, and at 980 nm, using injection laser diode pumps.
When pumping at 1480 nm, it is known to use a dual pump configuration in which one pump laser is coupled to one end of the amplifier fibre by means of a pump/signal wavelength multiplexing 2.times.2 coupler in co-pumping configuration, while the other pump laser is similarly coupled to the other end in counter-pumping configuration. For a number of applications it would be preferable to arrange to pump at 980 nm instead of 1480 nm. This may for instance be because of the greater electrical to optical conversion efficiency of amplifiers pumped at the shorter wavelength. A difficulty is however found in transferring the dual pump configuration, which has proved satisfactory in a system designed for pumping at 1480 nm, directly into a system designed for pumping at 980 nm.